1. Technical Field
Exemplary aspects of the present invention generally relate to a transmission device, a driving assembly including the transmission device, and an image forming apparatus including the driving assembly, such as a copier, a facsimile machine, a printer, or a multi-functional system including a combination thereof.
2. Description of the Related Art
Image forming apparatuses such as a copier, a facsimile machine, a printer, or a multi-functional system including a combination thereof generally include rotary members such as a development roller, a photosensitive drum, and so forth driven by a transmission device such as a gear and a timing pulley.
In order to facilitate an understanding of the related art and of the novel features of the present disclosure, as a comparison, a description is provided of an example of a related-art transmission device with reference to FIG. 10. In the related-art transmission device, rotation of a motor 360 is decelerated by a drive gear 390 and a driven gear 300 of a decelerator 370, and a shaft 380 coaxial with the driven gear 300 is rotatably driven, thereby rotatably driving the rotary members such as the development roller, the photosensitive drum. On both sides of a housing 371 of the decelerator 370, a pair of shaft bearings 372 is coaxially disposed to support rotatably the shaft 380.
Torque from the driven gear 300 is transmitted to the shaft 380 by an engagement pin 381 such as a parallel pin and a spring pin. Such a configuration is proposed, for example, in JP-2002-266952-A. As illustrated in FIGS. 11A and 11B, the engagement pin 381 is fitted to a groove 312 formed in the diameter direction in the driven gear 300, thereby transmitting torque. The torque is transmitted by the engagement pin 381. Other known torque transmission structures include, for example, a substantially D-shaped or a rounded rectangular-shaped engagement portion at which the driven gear 300 and the shaft 380 are fitted, and a key and a keyways.
If, in the image forming apparatus, a vibration component of the motor 360, a vibration and a speed fluctuation component generated at the meshing portion of the gears 390 and 300 are transmitted to a driven side via the shaft 380, an image defect which appears as a horizontal streak in a toner image occurs. More specifically, the vibration generated at the meshing portion of a gear and a timing pulley causes easily such an image defect. In view of the above, the gear and the timing pulley need to be designed to produce less vibration and to prevent transmission of the vibration to the driven side even when the vibration is generated.
In a tandem-type image forming apparatus in which multiple toner images in different colors are transferred onto an intermediate transfer belt such that they are superimposed one atop the other, a load fluctuation occurs when the intermediate transfer belt contacts a recording medium intermittently at a secondary transfer portion at which a toner image is secondarily transferred onto the recording medium. Such a load fluctuation also causes an image defect with a horizontal streak or the like. In order to suppress the load fluctuation, stiffness of the transmission device in the direction of rotation needs to be enhanced.
In the known image forming apparatus, an elastic member is provided to the shaft or the gear to prevent the vibration from getting transmitted to the driven side. Such a configuration is proposed, for example, in JP-H09-114160-A. In this configuration, however, even when the elastic member suppresses transmission of the vibration and speed fluctuations to the shaft, the stiffness in the direction of rotation may be weakened. As a result, with the elastic member, if the load fluctuation occurs on the driven side of the shaft, the rotation speed may fluctuate significantly. In other words, it is difficult to prevent fluctuations in the rotation speed attributed to the vibration and the speed fluctuation on the drive side, while preventing fluctuations in the rotation speed attributed to the load fluctuation on the driven side.
The gear and the timing pulley made of resin are advantageous in suppression of the vibration at the meshing portion of the gear and also provide a greater degree of freedom in shaping or formativeness. However, strength, stiffness, and accuracy are not as good as ones made of metal. By contrast, the gear and the timing pulley made of metal are advantageous in strength, stiffness, and accuracy. However, vibration generated at the meshing portion of the gear tends to be greater than ones made with resin. As a result, either the gear or the pulley made of resin or the gear and the pulley made of metal may result in unevenness in a resulting image due to the vibration and the fluctuations in the speed in an image forming unit.
To address such a difficulty, in one approach, two or more gears are made with different materials. Such a configuration is proposed, for example, in JP-4435093-B2 (WO2005/046957A1). In this configuration, the gears are made of metal coated with resin, thereby enhancing strength, stiffness, shock resistance, wear resistance, durability, and noise reduction.
If the transmission device of FIG. 10 is constituted of two parts such as proposed in JP-4435093-B2 (WO2005/046957A1), a compound gear constituted of a metal core 301 and a resin gear 302 such as shown in FIG. 12 may be employed, for example. Such a compound gear may cause deformation at the meshing portion at which the metal gear and the resin gear mesh and a reduction in torque resistance at a press-fit portion due to a difference in the coefficient of linear thermal expansion between the metal and the resin.
Furthermore, the coaxiality between the resin gear 302 and the shaft 380 may be reduced by the metal core 301 disposed between the resin gear 302 and the shaft 380. As a result, the compound gear illustrated in FIG. 12 may worsen the fluctuation of rotation of the gear as compared with the configuration illustrated in FIG. 10.
In view of the above, there is a demand for a configuration capable of preventing fluctuations in the rotation speed caused by load fluctuations on the driven side by enhancing rotation stiffness with at least two transmission parts such as a driven gear while keeping the two parts coaxial relative to the shaft with ease.